1. Field of the Invention
This invention relates to upgrading of hydrocarbon residuum streams, and more particularly to a process for simultaneously processing straight run vacuum resid with a cracked residuum.
Upgrading of hydrocarbon residua is a processing problem in both conventional petroleum refining operations and tar sand bitumen processing plants. The problems associated with residua processing are particularly acute in those refineries where geographic location does not permit sale of coke or fuel oil. Asphalt production often encounters severe market limitations that are aggravated by seasonal variation.
Although straight run vacuum resids are often considered the primary residuum that must be considered for upgrading or disposal, cracked residual stocks produced in conventional refining processes must also be included in the overall refinery economics. Such cracked residual stocks include FCC residues (decant oil, clarified oil, slurry oil), thermal tars, pyrolysis tars and other similar materials. As used herein, the term "resid" is intended to mean straight run vacuum tower bottoms, while cracked residual stocks are referred to collectively as cracked residua, or cracked residuum in the case of a single cracked material.
Cracked residua present a unique problem. They are highly aromatic, often contain suspended solids and usually are higher in sulfur than virgin stocks of the same boiling range. The aromaticity precludes their processing in zeolite catalyzed cracker units. The suspended particulates (catalyst fines or coke particles) present in these materials make direct catalytic hydroprocessing impractical due to catalyst deactivation and bed plugging problems. The relatively high sulfur content of most of these streams make direct sale as fuel impossible due to environmental constraints.
The process of this invention employs a variation of hydrogen donor diluent cracking to both reduce the quantity of residual material that must be disposed of as fuel and/or asphalt and to convert the cracked residua into an upgraded FCC feed or a low sulfur distillate fuel. The inherent aromaticity of cracked residua is used to advantage.
2. The Prior Art
The most pertinent prior art is believed to be U.S. Pat. No. 2,953,513 to Langer. The Langer patent describes the basic hydrogen donor diluent cracking (HDDC) process, and includes a detailed description of applicable operating conditions. The Langer patent further describes one and two stage HDDC operations utilizing vacuum distillation of the donor cracked material.
The Langer patent, however, does not suggest co-processing of straight run vacuum resid with a cracked residuum, and does not describe a process in which a cracked residuum such as decant oil from an FCC unit can be utilized as part of the donor diluent without subjecting the hydrogenation catalyst to contamination from solids entrained in the decant oil. In the process of the invention, cracked residua can be co-processed with straight run vacuum resids without subjecting the hydrogenation catalyst to contamination from solids in the cracked residua.